The present invention is related to the field of motor controllers.
Motor controllers are employed to control the rotational speed of a motor. One type of controller relies upon a measurement of the back electromagnetic force (back EMF) appearing across a motor coil to obtain an indication of motor speed. In theory, the back EMF value is proportional to the motor's speed. The controller compares the back EMF measurement to a velocity command signal in order to sense deviation of the motor's actual speed from the desired speed, and in response adjusts the drive applied to the motor to correct for the speed deviation.
In reality, the back EMF measured across a motor coil is not perfectly proportional to the motor's rotational speed. One factor responsible for the imperfection is the non-zero series resistance of the motor coil. While the ideal motor coil has zero resistance, a real coil can have several ohms of resistance. This is true, for example, in voice-coil motors commonly used as actuators for read/write heads in computer disk drives. When a non-zero current is flowing in the coil, a non-zero voltage component equal to the product of the current and the resistance appears in the measured back EMF. The measured back EMF, then, can be viewed as the sum of an this error component and an ideal back EMF to which the motor rotational speed is proportional.
In some motor controllers, the presence of this error in the back EMF measurement is ignored. In these controllers the error is safely ignored because the resulting error in motor speed is within specified speed deviation limits for the application in which the motor is used.
In some applications, however, it is desirable to more accurately control motor speed. In such applications the back EMF measurement error is unacceptable. One example is the case mentioned of a voice-coil motor for a head actuator. It is important to accurately control the speed of a read/write head as it is being loaded onto a disk, so that the head does not strike the disk hard and cause damage. Similarly, it is important to avoid striking the head against a head stop when retracting the head from the disk. In applications like a head actuator in which greater motor speed accuracy is required, it is necessary to minimize the error component from the back EMF measurement.